A baby who survived surgery to separate her from her conjoined twin remains "a very sick little girl", a children's hospital said.
Doctors at Great Ormond Street Hospital in London said they were giving Faith Williams the "highest possible levels of care".
The baby's sister Hope died after surgery to part them, in what the hospital called "one of the most complex and challenging operations" its staff had faced.
Hope failed to survive after Tuesday's procedure because her lungs were too small to support breathing, doctors said.
In a statement, Great Ormond Street said: "Faith Williams remains a very sick little girl. Great Ormond Street Hospital is doing everything it can for her, and we continue to offer her the highest possible levels of care."
The twins were delivered by Caesarean section at London's University College Hospital on November 26 with a combined weight of 4.8kg (10.5lb), and transferred to Great Ormond Street.
They were joined from the breastbone to the top of the navel and had a shared liver but separate hearts.
The girls' mother, 18-year-old Laura Williams, from Shrewsbury, Shropshire, is Britain's youngest mother to give birth to conjoined twins. The condition, which is extremely rare, occurs when the single egg from which identical twins develop fails to divide properly after conception.
Medical staff warned Mrs Williams and her husband Aled, 28, that their babies might not survive after a 12-week scan revealed the problem but they refused to consider a termination.
Doctors had hoped to wait until the twins were older and stronger to operate, but head of the surgical team Professor Agostino Pierro said surgery was brought forward because of a "sudden deterioration" in the children's health.
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